There were was no ideally about it - the tender stated compatible. So compatible they shall be.
The exact wording was:
preferably operationally compatible with existing London Midland vehicle fleet.
The tender also mentioned 2/3 door layouts, capable of operating in multiple, 100mph top speed, 3 or 4 carriage formation and high level of acceleration. In other words London Midland wanted 350s but because they had to issue a new tender they made it difficult for anyone other than Siemens to be awarded the bid.
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Hence the gap for a Crewe - Lancaster / Preston stopper when NT becomes flush with 319s that no-one else wants.
I very much doubt Northern will have 319s spare. If you consider there's some Northern Pacer services operating under the wires in the Manchester area at peak times as it is and the number of 319s Northern and FGW will require then it only leaves a few spare. I think if Northern do get those 'few spare' they'll be forced to give up a few 321/323s.